“(Iblees said) Because you (O Allah!) have sent me astray, I will sit in ambush for them (humans) on the Sirathal Musthaqeem (Your straight path)”.
O brother repent, repent, and repent because the doors of forgiveness will close when you die. And don't do it again. There are practical steps which reduces risk of falling into adultery (or fornication):
- Lower your gaze.
- Don't stare at non-mahram woman.
- Observe salat (because it stops from shamelessness; condition: must be performed properly to be effective).
- Observe fasting as it reduces sexual desires ( optional: marry 2,3 or 4 wives if you can do justice but never commit zina).
- Eat less food (reduces sexual power).
- If you feel sexual urge, then satisfy your desires immediately with your wife (if you can) as she is your protection from zina.
- Drink water, as water extinguishes fire from the devil.
- Avoid intermingling with opposite gender unless a necessity and restrict talking unless necessary.
- Don't shake hands with non-mahram (unless unavoidable). The punishment according to a narration is that a hot coal will be placed in the person's hand on that Day!
- Know that Allah is merciful but He is also Al-Khāfiḍ (the humiliator) and Al-Ḥakam (the judge). So avoid invoking wrath of Allah. We become so indulged that Allah will forgive us that we take His mercy for granted.
- Ask Allah for protection and always ask Him to protect you from shamelessness. Verily another one of His attributes is Al-Muʾmin (the granter of security).
I have, if not all, covered most of the steps that might help you, inshallah in not committing the sin again.
From Abu Masood (ra) who said that the Prophet said: "Among that which reached the people from the words of the earlier prophethood: If you feel no shame, then do whatever you wish." (Bukhari)